Calibre adds flow-specific CSS to centre cover images (@page style rules), and since v0.1.4 Sigil doesn't copy flow-specific CSS from the loaded epub. This was necessary since CSS conflicts arose, and the epub could get screwed up.
This will be fixed in v0.2.0, since that version will introduce a multi-flow editing paradigm.
For now, just select the image and press the "Centre" button.
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